Man made cellular solids often have useful strength to weight ratios and find applications as load bearing or energy absorbing products. Cellular metals are usually called metal foams. They consist of a network of interconnected solid struts or plates that form the edges and faces of cells. They can take the form of a honeycomb, open cell foam or closed cell foam.
Honeycombs consist of a two-dimensional array of polygons expanded in one preferential direction. The cells of the honeycomb are usually open in the preferred direction but the polygonal walls close the structure in other directions. Open cell foams consist of a network of open struts connected to one another with no cell walls. Open cell foams are made of cell edges only and they have an “open” structure through which a fluid could flow. Closed cell foams have cell walls that are continuous. The space within the cell walls is totally enclosed, containing only air or gases but there is no open passage between cells.
Closed cell metal foams with their empty cells and structural walls offer a very useful combination of reduced weight and strength. Ideally, they could be formed as an assembly of uniformly shaped and sized polyhedrons. According to engineering analysis such idealized metal foams would provide excellent strength and energy absorbing properties. But it has proven very difficult in practice to manufacture such geometrically regular cellular metal structures.
U.S. Pat. Nos. 4,973,358; 5,112,697 and 5,334,236, each assigned to Alcan International Limited of Montreal, Canada, describe methods and apparatus for making lightweight, closed cell foamed metal slabs. These disclosures describe a practice applied to aluminum alloy A356 containing, for example, 15 volume percent, finely divided (e.g., 0.1 to 100 μm in largest dimension) solid particles, such as silicon carbide particles, that are required for forming a stabilized foam. Air bubbles were discharged beneath the surface of the molten composite-alloy to produce a closed cell foam of the composite particulate/aluminum alloy material.
Foaming was accomplished using a movable air injection shaft into the liquid at an angle of, e.g., 30° to 45° to the horizontal surface. Several examples of foaming gas introduction using rotating or reciprocating gas injection shafts are described, especially in U.S. Pat. No. 5,334,236. The air or gas injection caused foaming of the molten composite above the point of gas discharge and agitation. The stabilized foam was removed in solid form from the surface of the molten composite. The foam was described as having cell size that was controlled by adjusting the air flow rate, the number of nozzles used in air injection, the nozzle size, the nozzle shape and the impeller rotational speed.
Aluminum foams of various densities produced by the described process are available from Cymat Aluminum Corporation of Mississagua, Ontario. However, Cymat foams have not been characterized by uniform cell structure. Purchased foams have the surface appearance of FIG. 10 of U.S. Pat. No. 5,334,236, but they have a defect-riddled porous structure. They are more like low-density porous metals than true cellular metal foams.
It is an object of this invention to provide a process of making a closed cell metal foam having a cross section characterized by a regularity of uniform cells with smooth concave walls that intersect at clearly defined boundaries.